The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
Methane, CH4 is the primary component of natural gas. It is both a valuable source of energy and potent greenhouse gas, with 21 times the global warming potential of carbon dioxide, CO2. Nitrogen, N2, is a common impurity in natural gas, varying from 0.5% to 25% by volume, depending on the source. This impurity has no energy content, no global warming potential, and needs to be removed from the natural gas to varying degrees to meet product sales specifications. Its removal is particularly important in the production of liquefied natural gas (LNG).
The separation of N2 and CH4 is challenging because they are molecules with very similar physical properties such as molecular size (0.364 and 0.380 nm respectively) and normal boiling point (−196 and −161° C., respectively). Conventionally, the removal of N2 during LNG production is performed through cryogenic distillation, which is an expensive and energy intensive process. The removed N2 must be disposed of and generally this is done by venting the N2 to atmosphere. However, the purity of this vent is limited by the similarity of the physical properties of N2 and CH4. A N2 vent stream containing about 2% CH4 can be achieved with a single stage of cryogenic distillation. However, the magnitude of the vent stream (˜50 tonnes/hour per 5 million tonne per annum LNG train) means that such a CH4 concentration corresponds to a significant amount of wasted energy and source of greenhouse gas emissions.
Adsorption technologies based on molecular diameter differences have been attempted for CH4 capture. However, separation of CH4 and N2 is very challenging because of their very close molecular diameters (3.80 Å vs 3.64 Å).